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How to choose TVS diodes to dispel flyback surges from relay coils?
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BravoV:

--- Quote from: calzap on February 05, 2020, 03:27:07 pm ---... slow the opening of the contacts leading to arcing and premature wear. ...

--- End quote ---

On preserving contacts ... abit related, a cap (+resistor) across the switch form a snubber that suppresses arc and prolong relay contact life -> Keysight AppNote : Maximizing the Relay Life Span

PS : It used to be a direct download for the "5988-6917EN.pdf", now Keysight needs registration.  ::)


Edit :

Alternate download link ->

https://www.newark.com/pdfs/techarticles/agilent/MaximizingtheLifeSpanYourRelays.pdf
T3sl4co1l:
Heh, interesting case I'm working with, a customer has relay contacts that often switch inductive loads (larger relays, solenoids) on a 24V DC system.  They're getting random resets when the contacts open.  Okay, sounds like EFT, let's put an RC across them.  Wait, there's already 4.7R + 1uF there?  That should slow things nicely...

As it happens, the 1uF's are failing shorted, possibly from manufacture (we've had issues with probably-cracked caps elsewhere on the board), but much more likely it's from overvoltage.  The solenoids are probably something like 10mH, so at a couple amperes, they store a good slug of energy, compared to that poor ceramic chip cap.  The 4.7R then burns open because it can't handle the load current.  End result, no RC, contacts arc.

All of this is made more awkward on account of the assembly being fully potted. ;D

Should be fine with something that can absorb the energy -- a TVS or MOV.

In short, the biggest problem with an RC (or other snubber network), the value must be suited to the load.  If you can't account for the worst-case load, you will eventually encounter problems.  (For AC loads, part of the limitation is leakage across the switch -- sure you can put on like 1R + 100uF, but it's practically never going to open.  The leakage eventually goes away for DC loads, but the inrush as the RC discharges through the contacts can be just as problematic!)

Conversely, a TVS or MOV is inflexible with respect to voltage.  In an automotive environment, you might be fine with a 18V TVS, in the average case, but what about load dump?  At mains voltage, you might be fine with a 360V MOV, but what about surge?  If it's acceptable for the load to turn on only momentarily, you can place the limiter across the contacts.  If not, you may need transient limiting at the source as well as contacts (a trio of MOVs between source, load and contacts, perhaps), or something more complicated to address the situation.

Tim
Yansi:

--- Quote from: NiHaoMike on February 06, 2020, 02:22:04 am ---For an AC relay coil, add a capacitor in parallel. One that gets the relay+capacitor close to a unity power factor is a good start.

--- End quote ---

Pretty bad idea, actaully - at least for the other contacts switching the coil with the cap in parallel.

Use series RC instead.
calzap:
trobbins,

Motor is 240 VAC, 1 Ph, 3 HP, direct-to-mains via 2 poles of the ACI contactor.  A description of a failed example of the motor is here:
https://www.eevblog.com/forum/projects/pump-motor-postmortem/msg2856420/#msg2856420

Motor is driving a two-stage centrifugal pump ... usually starting against 45 PSI pressure.  In-rush for ACI contactor is 70 VA.   System receives heavy use 8 months of the year.  In 11 years of operation, just about everything has failed at least once.  Biggest problem was repeated failure of the line voltage monitor that protects other controller components and  the pump.   Pretty much solved that by increasing ventilation of cabinet and increasing spike protection on the line-side of the voltage monitor.  But you can only do so much to shield from the sins of the power company.

Mike in California

trobbins:
Hi Mike, always good to appreciate the scope of the application, and not just zero in on a particular point.

When you say in-rush of contactor is 70VA, are you referring to the contactor coil or the 2 pole contacts that feed the motor?   Have you assessed or measured the motor full-load or locked rotor current, and related that to the contactor contact rating for say AC3 specification ?

As a first-in comment, it may be that the contactor contacts are the weak link in the chain and no tweaking of how the coil is managed will remove that weak link, or significantly alleviate the stress on the contacts (other than to insert an electronic motor drive in to a problematic high temperature environment).

Are you using a mains monitor relay with contacts that are used to turn off the contactor coil (and hence the motor) and other sensitive devices when mains AC voltage is either too high or too low ?  I've come across differences in how much internal power dissipation occurs in different such monitors (especially at 60Hz), and the max ambient temp rating for the devices - and had to swap over to another brand for 60Hz operation in one application.

Ciao, Tim
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